What do you all think of show-offs at the pool?

Did he move to the outermost lane on his own, or did he only do it after you looked at him? That detail is pretty important.

If there's a big splash but hardly any forward movement, the safer approach is to mention it to the front desk or a lifeguard first and let them decide whether to say something—provided someone is on duty and you'd rather not get into a direct confrontation. You could also go over and say something yourself, but since the pool is quiet early in the morning, it's best to make sure the person isn't deliberately looking for trouble before you speak up.

Wearing a full set of hand paddles and fins but still doing the doggy paddle—now that's quite the contrast.

When you say "innermost" and "outermost" lanes, which side of the pool are you referring to? If everyone assumes the inner lanes are the fast lanes, that rule may just never have been written down—it's not necessarily something everyone knows.

Look at it from another angle: when the pool is quiet in the early morning, lane assignments aren't always clearly defined, and some people figure an empty lane is fair game. But it varies a lot from place to place—some gym pools mark fast and slow lanes, while others don't mark anything at all and rely on regulars' unspoken understanding. If there's no clear signage here, his choice may not have been a deliberate attempt to crowd you out; he may simply not have realized he should leave the middle lane open.